Recently, interests in energy storage technology have been gradually increased. As the use of batteries is enlarged to applications for the storage of energy for portable telephones, camcorders, notebook computers, personal computers and electric vehicles, efforts on the research and development of batteries are increasingly embodied. In this view, the field of electrochemical devices receives the greatest attention, and among them, interests in the development of chargeable/dischargeable secondary batteries are focused. More recently, in the development of such batteries, active studies have been conducted to design a novel electrode and battery, which provide an improved capacity density and specific energy.
Among secondary batteries which are now in use, lithium secondary batteries developed in the early 1990s are in the spotlight due to the advantages of higher drive voltages and far greater energy densities than those of conventional batteries, such as Ni-MH, Ni—Cd and sulfuric acid-lead batteries. In general, a lithium ion battery comprises a lithium metal oxide as a cathode active material, a carbonaceous material or a lithium metal alloy as an anode active material, and a solution containing a lithium salt dissolved in an organic solvent as an electrolyte. Organic solvents that have been used widely in recent years include ethylene carbonate, propylene carbonate, dimethoxyethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofuran or acetonitrile. However, the organic solvents have enough volatility to cause evaporation, and are also highly ignitable, and thus are problematic in terms of stability under overcharge, overdischarge, short circuit and high temperature conditions, when applied to a lithium ion secondary battery.
Recently, there has been an attempt to use an ionic liquid as an electrolyte in order to solve the above problems. U.S. Pat. No. 5,827,602 (V. R. Koch et al.) discloses an AlCl3-EMICI (1-ethyl-3-methylimidazolium chloride) ionic liquid containing a strong Lewis acid useful as an electrolyte for a lithium ion battery. Since AlCl3-EMICI has no vapor pressure, it is possible to solve the problems of evaporation and ignition of an electrolyte. However, when AlCl3-EMICI is exposed to a small amount of water or oxygen, it emits harmful gas. Also, AlCl3-EMICI has additional problems in that it has high reactivity with electrolyte additives and is easily decomposed particularly at a temperature above 150° C.
As another attempt, Japanese Patent Publication No. 2002-110225 discloses an imidazolium-based ionic liquid and an ammonium-based ionic liquid for use in a lithium ion secondary battery. However, the ionic liquids are problematic in that they are reduced at an anode under a voltage higher than the redox potential of a lithium ion, or imidazolium and ammonium cations may be intercalated into an anode along with lithium ions. Further, even if either an imidazolium-based ionic liquid or an ammonium-based ionic liquid is used alone as a liquid electrolyte for a lithium secondary battery, the ionic liquid is not suitable to be applied to a practical secondary battery due to a significant drop in capacity of a secondary battery during repeated charge/discharge cycles.
Therefore, many attempts have been made to modify an electrode active material or to develop a novel electrode active material so that the problems occurring in conventional organic electrolytes and ionic liquids can be solved. Also, various attempts have been made to develop a novel electrolyte comprising additives.